Membrane processes useful for the separation of gaseous mixtures employ large differentials in chemical potential, usually applied as a pressure gradient, across a membrane to drive separations. On the permeate side of the membrane, low pressure is usually maintained by the use of compressors, vacuum pumps, or low temperature condensers. On the feed side of the membrane, the driving force is kept high by using high pressure or high temperature.
Membranes useful for the separation of gaseous mixtures are of two very different types: one is microporous while the other is nonporous. Discovery of the basic laws governing the selectivity for gases effusing through a microporous membrane is credited to T. Graham. When the pore size of a microporous membrane is small compared to the mean-free-path of non-condensable gas molecules in the mixture, the permeate is enriched in the gas of the lower molecular weight. Practical and theoretical enrichments achievable by this technique are very small because the molecular weight ratios of most gases are not very large and the concomitant selectivities are proportional to the square roots of these ratios. Therefore, a large number of separation stages is needed to effect an efficient separation of a given gas from a gaseous mixture. However, because this method of separation relies solely on mass ratios and not chemical differences among the effusing species, it is the only membrane based method capable of separating isotopes of a given element. For this reason, this method was chosen to enrich uranium in the fissionable isotope 235 for development of the atomic bomb during World War II. However, this method of separation is inherently expensive due to the large amount of capital investment needed for processing a necessary large amount of gas, stringent membrane specifications requiring high porosity and small pore size, and high energy requirements for operation.
In nonporous membrane systems, molecules permeate through the membrane. During permeation across the nonporous membrane, different molecules are separated due to the differences of their diffusivity and solubility within the membrane matrix. Not only does molecular size influence the transport rate of each species through the matrix but also the chemical nature of both the permeating molecules and the polymer matrix itself. Thus, conceptually useful separations should be attainable.
Vapor permeation is very closely related to membrane gas separation, as pointed out by Gas separation is one of the largest applications of membrane technology. For example, see Lee and Koros in “Membranes, Synthetic, Applications” published in the Encyclopedia of Physical Science and Technology, Third Edition, Volume 9, Academic Press (2002).
Membrane based technology for the production of nitrogen from air, removal of carbon dioxide from natural gas, and purification of hydrogen occupy significant shares of the markets for these processes. Most of the technical challenge for membranes for these applications has been in developing membrane materials that can selectively remove the desired component. A number of patents that have been issued for specific membrane materials, however little attention has been given to the heat balance around the membrane apparatus, primarily because components previously considered for membrane based separations (nitrogen, oxygen, carbon dioxide, methane, hydrogen) are fixed gases. Such gases do not exist both as a liquid and a vapor at typical conditions of industrial process.
The art is replete with processes said to fabricate membranes possessing both high selectivity and high fluxes. Without sufficiently high fluxes the required membrane areas required would be so large as to make the technique uneconomical. It is now well known that numerous polymers are much more permeable to polar gases (examples include H2O, CO2, H2S, and SO2) than to nonpolar gases (N2, O2, and CH4), and that gases of small molecular size (He, H2) permeate more readily through polymers than large molecules (CH4, C2H4).
Pervaporation refers to a membrane process where the feed to the membrane is a liquid. High driving force is maintained by warming the liquid and keeping the permeate at low pressure. As material passes across the membrane, energy is transferred from the feed to the permeate. This loss of energy from the feed side tends to cool the feed and lower the membrane driving force. In order to reestablish a high driving force, the liquid must be reheated. In practice, this leads to staged membranes with interstage reheating. However, Rautenbach and Albrecht state in an article entitled “The Separation Potential of Pervaporation, Part 2: Process Design and Economics” published in Journal of Membrane Science, vol. 25, pp. 25–54 (1985) that the complexity of multi-stage pervaporation processes would make commercial application unfavorable.
There do appear to be cases where pervaporation is efficient enough to be practiced on an industrial scale. Baker states in a book entitled “Membrane Technology and Applications” published by McGraw-Hill (2000) that one of the largest applications of pervaporation is the dehydration of ethanol. Hendrikus et al. describe in U.S. Pat. No. 4,925,562 a pervaporation membrane useful for the permeation of several alcohols. Shucker et al. describe a multistage pervaporation process in European Patent Application Publication Number 0457981 A1. Pervaporation also appears attractive when employed in concert with other separation technologies. A review article entitled “Pervaporation-based hybrid process: a review of process design, applications, and economics” published by Lipnizki et al. in Journal of Membrane Science, vol. 153, pp. 183–22 (1999) examined several pervaporation membrane hybrids.
One way to keep the driving force high on the feed side of the membrane is to increase the energy of the feed stream so that energy losses due to permeation are not as significant. Adding energy to the feed so as to vaporize the feed results in a process called vapor permeation. There are very few descriptions of vapor permeation in the prior art. Friesen et al. describe a process useful to separate mixtures of vapors in European Patent Application Publication EP0701856A1.
For polymeric membranes, a large pressure gradient across the membrane would supply the driving force for permeation. This driving force would induce a cooling in the membrane (for materials with positive Joule-Thomson coefficients) in order to produce the low pressure permeate. This affect is not present in facilitated transport membranes and has not been incorporated in previous processes based on them. Most of this work focused on details of the internals of the facilitated transport membrane device and not on how to incorporate them into a process that produced products that met market specifications.
Some of the most difficult separations in the petrochemical industry involve the separation of one isomer of an aromatic compound from another and/or other organic compounds, for example isomers of xylene and ethylbenzene. The separation and purification of para-xylene (PX) from mixed xylenes is an energy and capital intensive process. Industrial processes used today employ energy-intensive cryogenic separations or capital-intensive absorbent technology to produce high purity PX. It is widely recognized that, next to feedstock costs, the purification section is the most expensive part of the para-xylene production.
There is, therefore, a present need for processes and apparatus using perm-selective membranes to provide heat integrated membrane apparatus where pressure-driven (fugacity-driven) membranes for the separation of selected compounds from mixtures which when subjected to appropriately altered process conditions of temperature and/or pressure exhibit a bubble point. Advantageously, a new process should overcome the recovery limitation imposed by membrane cooling encountered in pervaporation.
Improved apparatus should provide for an integrated sequence, carried out with streams in gas and/or liquid state, using a suitable perm-selective membrane, preferably a solid perm-selective membrane which under a suitable differential of a driving force exhibits selective permeability of a desired product, i.e., incorporate pressure-driven (fugacity-driven) membranes with existing separation assets.